To calculate the force one person exerts on the other during the collision, you would need:
the mass and acceleration of each person.
This is because, according to Newton's second law of motion (F = ma), force is determined by the mass of an object and its acceleration. In the context of a collision, you would need to know how fast each person was moving (which gives you a change in velocity, and thus acceleration) and their respective masses to calculate the forces involved accurately.